An Adobe patch for a well-publicized bug in the company's Reader PDF software doesn't fix the vulnerability, the security researcher who uncovered the flaw confirmed.

Last Tuesday, Adobe shipped an update for Reader and Acrobat, its popular PDF viewing and creation programs, that patched 17 vulnerabilities, including a design issue that gave attackers an easy way to con users into running malware. The bug, which was disclosed by Belgium researcher Didier Stevens in late March, allowed hackers to leverage the "/Launch" function, a feature that executes other software from within a PDF document.

When combined with another trick -- Stevens also showed how a Reader warning could be changed to further fool users -- the flaw could be used to dupe users into launching malware masquerading as legitimate software.

Hackers have been using Stevens' technique in mass attacks to infect Windows PCs since mid-April.

On Sunday, Stevens confirmed that the patch didn't fix the flaw.

"I did some research and discovered that Adobe implemented a blacklist of extensions for the launch action, but that the blacklisting functionality identifies the file type of 'cmd.exe' as .exe', and not .exe," he said in a post to his blog July 4.

Attackers can still get a PDF document to run malware simply by enclosing the malicious filename within single- or double-quotation marks, Stevens said.

Le Mahn Tung , a researcher at Bach Khoa Internetwork Security (BKIS), which is housed at the Hanoi University of Technology in Vietnam, first noticed that Adobe had not properly patched the bug.

On his blog, Stevens offered a workaround to deflect attacks that sidestep Adobe's blacklist. His workaround, however, involves editing the Windows registry, a task most users will shy away from.

Adobe admitted its blacklist wasn't foolproof, and said it was looking into Tung's and Stevens' bypass techniques.

"While blacklist capabilities alone are not a perfect solution to defend against those with malicious intent, this option reduces the risk of attack," said Brad Arkin, Adobe's director of security and privacy, in an entry on the company's security team's blog . "We will evaluate [the Tung] workaround to determine whether additional changes to the blacklist are required."

Adobe fixed the warning dialog flaw, Stevens noted, so that hackers could not modify it to increase the chance that users would fall for their ploys. Adobe's Arkin also pointed that out. "The warning message provided includes strong wording advising users to only open and execute the file if it comes from a trusted source," Arkin said.

Last week's patch also modified Reader and Acrobat so that the /Launch feature is disabled by default, requiring users who want the functionality to manually switch it on. In earlier editions, /Launch was enabled by default.

The updated Reader and Acrobat for Windows, Mac and Linux can be downloaded using the links included in the security advisory Adobe published last week. Alternately, users can run the programs' built-in update mechanism to grab the new versions.